1. Field of the Invention
This invention relates to laser diode transmitters of the type used in fiber-optic communication systems and more particularly is directed to compensating circuits for correcting the light output of the laser diode against variations caused by changes in laser diode temperature.
2. State of the Prior Art
Semiconductor laser diodes are widely used as optical sources in fiber-optic communication links. The laser diode is driven by an alternating current (a.c.) which represents the information to be transmitted over the optical fiber. The optical output of the laser diode, now also carrying the desired a.c. signal, is coupled to a corresponding optical fiber which conveys the optical signal to a receiver coupled to the opposite end of the fiber, where the transmitted a.c. signal is recovered in electrical form for further processing. A given transmitter package may include one or several such laser diodes, each with a corresponding diode driver circuit and coupled to a corresponding optical fiber.
Laser diodes have a characteristic threshold current which must be exceeded before any light output is emitted by the diode. In typical transmitter packages the laser diode is supplied with a bias current which places the diode at the threshold of optical emission. This is done in order to maintain linearity and signal strength of the a.c. signal being transmitted. The a.c. signal supplied by the diode driver circuit may be any signal varying in amplitude over time, although in digital communications it normally consists of a digital pulse train. The a.c. drive current input to the laser diode results in emission of a light output having a time-varying characteristic, typically the amplitude or intensity of the light output, representative of the a.c signal and carrying the desired information.
Semiconductor laser diodes are sensitive to changes in temperature of the environment in that the threshold current increases with rising temperature while the output level or intensity of the emitted light decreases with rising temperature. The temperature variations of concern are changes in room temperature and/or heating in the instrument housing containing the laser diode transmitter. If no correction is made for this effect, the result is distortion of the a.c. signal carried by the light output as well as diminished overall intensity of the light output. Both these consequences impair the quality of the communications link and may result in outright failure of the link if the light output of the laser diode falls below the minimum level detectable by the receiver at the other end.
Much effort has been expended in devising means for correcting for this temperature susceptibility of laser diodes. The conventional approaches broadly fall into two categories: optical feedback and active cooling. The optical feedback approach involves actually sensing the light output of the laser diode with a photo-detector, and connecting the output of the photo-detector for increasing drive current to the laser diode to compensate for diminished output with rising temperature. Active cooling calls for refrigerating the laser diode by such means as Peltier junction devices in order to hold constant its temperature. The former approach fails to correct for changes in threshold current of the laser diode, while the latter approach consumes excessive power.
Attempts have also been made to adjust the bias and drive currents as a function of temperature to compensate for temperature induced changes in the corresponding laser diode operating characteristics. However, the changes in threshold current and drive current requirement as a function of temperature vary in a manner which can be generally approximated by an exponential function. Known efforts along these lines have relied upon microprocessor systems equipped with tables of stored values representing the diode currents at closely spaced temperature points through the operating temperature range of the laser diode. This approach requires digital memory as well as a microprocessor and supporting circuits, leading to undesirable complexity and excessive power requirements.
A continuing need exists for a more efficient approach to the temperature compensation of laser diodes.